In "The Lottery", explain the role of mass psychology in the decision of the townspeople to participate in the stoning.

First of all, the mindset of the townspeople is extreme conservatism, in a basic sense.  They submit to the Lottery because their ancestors did.  It had always been done.  In a sense, it's a tradition. 

Also, they saw some economic benefit in it.  Without it, there was no assurance that crops would grow.  It was purely self-interest in this respect.

They were aware that other communities had altered their practice of the Lottery, but as a whole, were not willing to alter their own.

And at the most elemental level, violence breeds violence.  They wer caught up in the mass hysteria of the stoning.  It gave them a sense of participation in the community and in insuring the community's survival.